Rotor assemblies are used in various systems, such as gas turbine engines and turbochargers. In a gas turbine engine, pressurized air that is produced in a compression system is mixed with fuel in a combustor and ignited, generating hot combustion gases which flow through one or more turbine stages. The turbine stages extract energy from the hot combustion gases for generating engine thrust to propel a vehicle (e.g., a train, an aircraft, a marine vessel, etc.) or to power a load, such as an electrical generator.
The gas turbine includes a rotor assembly having a plurality of blades extending radially outward from a rotor disk. Each blade has a mounting segment, such as a dovetail, that engages the disc, and an airfoil extending from the mounting segment to a tip of the blade. In some rotor assemblies, the blades have at least one pair of shrouds on the airfoil. In each pair, one shroud extends from one side of the airfoil and the other shroud extends from an opposite side of the airfoil. The shrouds are located along a length of the airfoil between the tip and the mounting segment of the blade (e.g., mid- or part-span shrouds) and/or at the tip of the blade (e.g., tip shrouds). During normal operation of the compression system, the blades twist as the rotor assembly rotates and the shrouds on adjacent blades contact each other, forming a circumferentially-extending shroud ring that provides support to the blades. The shroud ring couples the blades together through friction at the shroud interfaces to dampen vibration of the blades to mitigate high cycle fatigue risk. Reducing or eliminating vibrations of the blades can extend the useful life of the turbine blades.
There are some disadvantages associated with the conventional shrouded turbine blades that provide damping through friction at the shroud interfaces. For example, in order to provide appropriate sliding and energy dissipation between the shrouds, the contact load at the interface between the shrouds of adjacent blades needs to be maintained at an appropriate level, which requires precise machining and assembly of the rotor assembly. The contact load is dependent on an initial shroud gap between shrouds of adjacent blades, amount of untwist of the blades during operation, and stiffness of the airfoils. Due to manufacturing tolerances, it may be difficult to control the contact load for effective damping over a wide operating range. Conventional shrouded turbine blades also have a potential for damage at low rotational speeds before the shrouds of adjacent blades engage each other, which is referred to as lock up. For example, to maintain a relatively low contact load between the shrouds at operating speeds, which is desirable for damping, the blades are designed with a relatively large shroud gap such that the shrouds lock up at a relatively high rotational speed. At speeds lower than the lock up speed, the adjacent blades are not connected to each other remote from the rotor disk, so the blades risk damage due to vibration, aerodynamic instability, and the like. Furthermore, the blades may experience high temperature creep over time which can significantly affect the contact load at the interface between the shrouds of adjacent blades. The creep therefore affects the amount of energy dissipated between the blades over time, which can make the blades susceptible to high vibratory stresses.